Physiologic concentrations of melatonin have been previously shown by us to increase estrogen receptor concentrations in vitro in a human breast cancer cell line. This in vitro model system is being investigated further to determine if this will be a reliable biological response and to assess what hormonal parameters affect the melatonin response of these cells to melatonin. The present data indicate that melatonin promotes estrogen-stimulated growth of MCF-7 cells implanted in nude mice and promotes tumor growth in vitro of insulin-stimulated MCF-7 cells.